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Stoat Trapping Trial Analysis Report

Iain Malzer, Bea Ayling, Carmen Biondo

Introduction:

Trapping trials have been undertaken in three phases for the Orkney Native Wildlife Project (ONWP) across three seasons: winter, summer and autumn. These trials were undertaken to assess the effectiveness of different combinations of trap type and bait, in key habitats, across different seasons in catching stoats on the Orkney Mainland as a prelude to a major eradication project planned for 2019.

The aims and results of the initial winter trial, conducted by contractors, SCL, can be found in a

separate report, however data collected in this trial was examined as part of this wider trapping trial

analysis report.

It was decided to undertake further trapping trials in summer and autumn to address some of the

shortcomings of the initial trapping trial carried out between December 2017 and February 2018,

and to collect trapping data during the summer season. This will serve two purposes: inform the HRA

with regards to disturbance to breeding birds, and to determine whether trapping rates differ when

natural food availability is greater, and females have dependant young.

The aims of both the summer and autumn trapping trials were to:

1. Test the attractiveness of alternative bait types to stoats: smoked fish, beef offcuts, dried

rabbit, tinned cat food and hens’ eggs – and synthetic lures.

2. Assess the effectiveness of trapping during the summer season when natural food is more

readily available

3. Assess the effect of season, trap type and habitat type on catch success.

Methods:

Stoats were trapped across three, roughly 5km2 zones, on the Orkney Mainland, west of Kirkwall:

Hobbister, Grimbister and Wideford. Within each of these zones, traps were set on a grid of 350m

resulting in 36 trapping locations per zone (in winter) and 18 (in summer and autumn). Traps were

not rigidly set at grid locations, but were placed along linear features such as walls or ditches to

maximise encounters with stoats. The traps were set almost equally across two major habitat types:

‘heathland’ and ‘fields’ (including both improved and unimproved grassland).

Trapping trial study areas: the number of trapping locations used during summer and autumn trials. In winter,

double the number of traps were used in each of these areas.

At each trapping location, effectively three traps were placed: two (double‐set) DOC150s in a double‐

entrance tunnel and one (single‐set) DOC200 in closed‐end tunnel.

The double‐set DOC150 double‐entrance tunnel set up was changed between trial periods. An internal

wire mesh barrier between the two traps (effectively creating two closed‐end tunnels within the same

box) was present during the winter and summer trials. This baffle was removed from all double‐set

traps in August 2018 following advice from Angela Newport (an expert trapper and stoat detection

dog handler) and the autumn trial was conducted without them present.

The single‐set DOC200 trap was placed in a shorter, closed‐end tunnel. The distance from the box

entrance to the trap was extended using a short section of drainage tube attached to the front baffle.

This design amendment was added to these closed‐end tunnels following an earlier incident prior to

the start of the trials when a cat caught its paw in a trap.

As mentioned above, three trapping trials were undertaken in total:

1. The winter trial took place between 7th December 2017 and 14th February 2018 and

involved 7 checks of the traps (undertaken by contractors SCL)

2. The summer trial took place between 4th June and 5th August 2018, involving 6

approximate weekly checks by RSPB/ONWP staff; and

3. The autumn trial took place between 11th September and 5th November 2018 and involved 6

approximate weekly checks by RSPB/ONWP staff.

The summer and autumn trials varied from winter trials in their use of baits and lures. During the

winter trial, a single type of bait was used, fresh skinned rabbit. For this reason, data on bait

preferences for winter trial are not available.

In the summer and autumn trials, a more systematic approach to baiting / luring was undertaken. Five

bait types, smoked fish, beef offcuts, dried rabbit, tinned cat food and hens’ eggs – and in the autumn

only, synthetic lures were used in rotation across all trapping areas.

Results and Analyses:

Trap encounters were low with 75 stoats caught over all three trials and thus the data are sparse.

Data can be analysed at the following levels:

The winter trial: this data can be tested for impacts of zone, habitat, trap type, and days since

trapping began.

The summer and autumn trials: this data can be tested for impacts of zone, habitat, trap type,

bait and days since trapping began.

Winter trial summary

In the winter trial there were 3 zones, 36 trap locations, effectively 3 traps (2xDOC150; 1xDOC200) at

each trap location and 7 trap checks. Each single trap in each trap round was recorded as a single line

of data, resulting in 2268 data points. Some traps were not set on a given round due to loss or

malfunction and so 27 lines of data which had 0 trap nights were dropped from the winter analysis.

In the winter trial, each of these had the related variables:

Zone: Hobbister, Grimbister, Wideford

Trap type: double‐set DOC150 in double‐entrance tunnel, single‐set DOC200 in closed‐end

tunnel

Habitat: heathland, fields

Lure peg used: Yes or No

Trap nights: integer, the number of days the trap was active in each round.

Time days: The number of days since trapping began in the given zone.

Stoat capture ‐ this was coded as a dummy variable: 1 for capture; 0 for no capture.

There was a total of 19,833 trapping nights in the winter trial, with each trap left out for a mean of 8.9

nights (min 3, max 21) per round. The total number of stoats captured in the winter analysis was 41.

However, it has only been possible to use 33 catches in the statistical analysis as the catch data from

the final round of trap checks were not provided by SCL.

Of these 33 stoats, 29 (88%) were caught in double‐set DOC150s in a double‐entrance tunnel (with

wire mesh partition between the two traps) and 4 (12%) were in the single‐set DOC200 trap in a

closed‐end tunnel. On three occasions, both DOC150 traps within the same box at a single location on

a single round had captured two stoats.

1.5% of checks to traps resulted in a stoat capture and the rate of capture in the winter trial trapping

night was 0.002. The rate of capture per trap night was 0.0006 in fields and 0.0026 in heathland. The

rate of capture per trap night declined with the trap round (see Figure 1)

Winter trial modelling approach

A logistic modelling approach within a generalised linear mixed model framework was used to model

the binary variable stoat capture as a function of the variables: zone, trap type, habitat, bait stick used

and time days. To model expected variation between the individual trapping locations, the GPS label

of the trapping location was included as a random effect. Time days coded as a categorical variable

was also fitted as a random effect in the initial models but did not explain much variation while tending

to prevent models converging and was therefore removed. To account for the fact that traps left out

for longer may have a higher success rate, the log of the trapping nights variable was included as an

offset variable.

A stepwise AIC approach was used to select the most parsimonious model from the various candidate

models as the random effect term did not change between models.

Table 1 shows the estimates and significance of the terms included in the model. The terms that

remained in the most parsimonious model are shown in bold.

The effect size of the categorical variables is shown relative to the base level. The time since the

trapping trial began in winter was a highly significant term showing a decrease in the probably of

capturing a stoat per day since trapping began (Figure 2). The trap housing type was also highly

significant and suggested an increased chance of catching stoats in the double‐set DOC150s in a

double‐entrance tunnel. The final model also showed an effect of habitat, where the traps in

heathland had a considerably higher chance of catching stoats but due to a wide standard error, this

term was not significant. The bait and zone terms were not useful in explaining the variation in the

probability of stoat captures.

Variable Estimate (SE) Z Value P Value

Time days ‐0.05 (0.01) ‐4.00 <0.001*

Trap type Double‐set DOC150 in

double‐entrance tunnel

1.58 (0.58)

2.71

<0.001*

Habitat: Heathland

1.65 (0.99)

1.66

0.095

Table 1: Winter trapping trial model outputs. Terms that remained within the final model are highlighted in

bold.

Figure 1: Model predictions of probability of capture per day since trapping began for double‐set DOC150 traps

in a double‐entrance tunnel (black) and a single‐set DOC200 in closed‐end tunnel (red).

Dotted lines are the mean + or ‐ SE

Summer trial summary

In the summer trial, the three same zones were trapped. However, for the summer trial, only half

the number of the winter locations and traps were used, resulting in 18 trap locations and again they

Zone Hobbister Wideford

‐0.43 (1.06) ‐0.43 (1.08)

‐0.41 ‐0.46

0.68 0.63

Lure peg use Y

0.86 (0.66)

1.3

0.20

were almost equally split between ‘heathland’ and ‘fields’. The raw stoat catch data collected can be

found in the Appendix.

In total, during the whole period of the trial from 04/06/2018 and 05/08/2018, there were 978

occurrences of traps being set. 12 of these lines of data had to be removed due to having 0 trap

nights.

In the summer trial, each of these had the related variables:



tunnel


Bait: egg, beef, cat food, smoked fish, dried rabbit


Time days: The number of days since trapping began in the given zone.

Stoat capture ‐ this was coded as a dummy variable: 1 for capture, 0 for no capture.

There was a total of 9454 trapping nights in the summer trial, with each trap left out for a mean of 9.7

nights (min 3, max 27) each round. The total number of stoats captured in the summer trial was 13.

10 (76%) of these were in the double‐set DOC150 traps in a double‐entrance tunnel, and 3 (23%) were

in single‐set DOC200 traps in closed‐end tunnels.

There were no instances in the summer trial of two stoats being caught in the double‐set DOC150

traps in a double‐entrance tunnel. 1.3% of trap checks resulted in a stoat capture and the rate of

capture per summer trial trapping night was 0.001.

Seven stoats were caught in heathland habitat and six in grassland (five of which were in rough,

unimproved grassland, near or adjacent to heathland). The one stoat caught in improved grassland

was on the coastal edge.

The rate of capture per trap night was similar in fields and heathland during the summer trapping trial

and the decline in captures per trap round was less marked than in the winter trial.

Summer trial modelling approach

The logistic mixed modelling approach was undertaken for the summer analysis was largely similar to

that undertaken for the winter trial. The major difference was the addition of the multi‐level factor

for bait types in the summer models. Initially, an interaction term was included between the trap type

and bait type but limited the ability of the models to converge. The models included a random effect

for trapping location which helped explain much of the variation that was not explained by the

modelled fixed effects.

Due to the lack of captures there was very little power with which to explain the variation in probability

of capture and standard error within the estimates was high. Thus, few terms were deemed

significant. The most effective model for the summer trial data suggested an influence of bait type,

where hen’s egg showed a higher chance of catching stoats than the other bait types and time days.

The trap type, habitat and zone were not useful in explaining variation during the summer trial and

dropped out of the final models.


Time days ‐0.06 (0.03) ‐2.06 0.04*

Bait type Cat food

Dried Rabbit Hen’s Egg

Smoked Fish

0.08 (1.66) 0.99 (1.66) 2.87 (1.33) 0.84 (1.62)

0.05 0.60 2.15 0.52

0.96 0.54 0.03* 0.60

Habitat Heathland

‐0.37 (1.15)

‐0.33

0.74


‐0.48 (1.07) ‐0.15 (1.13)

‐0.45 ‐0.14

0.65 0.89

Trap Type Double‐set

DOC150 in double‐entrance tunnel

0.75 (0.73)

1.01

0.31

Table 2: Summer trapping trial model outputs. Terms that remained within the final model are highlighted in

bold.

Autumn trial summary

The autumn trial was similar to the summer trial. The same zones and trap locations were used. The

raw stoat catch data collected can be found in the Appendix.

The same baits were used, though there was the addition of synthetic scent lures instead of a food

bait on 126 traps during the whole trapping trial period. The total autumn trapping period was from

14/09/2018 until 05/11/2018. In total there were 969 occurrences that traps were set. This is slightly

lower than the expected number (3 zones, 18 trap locations, 3 traps, 6 checks = 972) as three traps

were not checked on one round due them becoming temporally unavailable.

Each trap had the following related variables:



tunnel


Bait: egg, beef, cat food, smoked fish, dried rabbit, lure.


Timedays: The number of days since trapping began in the given zone during this trapping

trial.

Stoat capture. This was coded as a dummy variable: 1 for capture; 0 for no capture.

There was a total of 7812 trapping nights in the autumn trial, with each trap left out for a mean of

8.06 nights (min 2.5, max 22) per round. The total number of stoats captured in the autumn trial was

29. 25 (86%) of these were in double‐set DOC150 traps in double‐entrance tunnels and 4 (14%) were

in single‐set DOC200 traps in closed‐end tunnels. There were no occasions during the autumn trapping

trial that both DOC150 traps placed in the same double‐entrance tunnel box captured two stoats in

the same round.

3% of trap checks resulted in a stoat capture and the rate of capture per autumn trial trapping night

was 0.0037. Eight of the captured stoats were in fields, while 21 were in heathland (total capture rate

in fields: 0.0024; in heath: 0.0046). Of the eight caught in fields, four were in unimproved grassland

or adjacent to heathland, three were in improved grassland but on the coastal edge and one was in

improved grassland but along a linear farm track.

Figure 2 shows the trend in capture rates as the trapping rounds progressed. There was a decrease

over the first five trapping rounds but trapping rate increased to its peak on the 6th round.

Autumn trial modelling approach

A logistic mixed modelling approach was used again for the autumn trapping data. Bait types were

treated in the same way as the summer data, but an extra bait type ‘lure’ was added. There were a

number of different scent lures trialled but due to a low sample size these were all pooled into a single

‘lure’ level within the bait type variable. The models included a random effect for trapping location

which helped explain much of the variation that was not explained by the modelled fixed effects.

The most effective model for the autumn trial data suggested a strong influence of the smoked fish

bait, which was estimated to have around 1.5 times higher capture rate than the other bait types. The

raw data suggest 11 (37%) of the autumn stoats were captured with smoked fish. There was no

interaction between the zone and the bait. The trap type also had a significant term with the double‐

set DOC150 traps in double‐entrance tunnels having a higher capture rate. There was a weaker effect

of habitat, where trapping in the heathland habitats resulted in higher capture rates, but this was not

carried through into the final model based on AIC. Interestingly, in the autumn models there seemed

to be no impact of time days on capture rate suggesting that stoat capture rates did not significantly

reduce during over the trapping period (this is coherent with Figure 2 which shows a weaker decline

in capture rates over the trapping trials in the autumn period). Table 3 below shows the results from

the best model.


Bait type Cat food Dry Rabbit Hen’s Egg

Smoked Fish Lure

0.00 (0.82) 0.96 (0.69) ‐0.41 (0.92) 1.41 (0.66) ‐0.14 (0.67)

0.10 1.39 ‐0.45 2.13 ‐0.15

0.99 0.17 0.66 0.03* 0.88

Trap Type DOC150 in double‐entrance tunnel

1.22 (0.55)

2.23

0.06*

Habitat Heathland

0.75 (0.43)

1.75

0.08


0.12 (0.46) 0.07 (0.48)

0.26 0.15

0.80 0.88

Time days ‐0.01 (0.01) ‐0.84 0.40

Table 3: Autumn trapping trial model outputs. Terms that remained within the final model are highlighted in

bold.

Figure 2: Rate of capture per trap night over the seven trap rounds during the winter (black) and six trap

rounds in the summer (red) and six trap rounds in the autumn (green).

Discussion and limitation of trials:

There are more stoat captures in the winter and autumn trapping trials which allows more

useful modelling of the data from these seasons. Rates of capture in winter and autumn were

higher than in summer so this is not due to there being more trap nights in winter. Higher

food abundance in spring and summer could mean stoats are less likely to use traps. It could

also be that there is a preference for fresh rabbit bait, but it was only used during winter so

we cannot compare these data.

There is a trend of declining captures since trapping began in winter and (albeit less marked)

in summer. This suggests there is an impact of trapping on the local population. This is not

the case in autumn when the capture rates decline but then increase again on the final round.

This could be due to the possibility of dispersing animals and an increased number of animals

moving into the trapping area.

The analysis shows that the double‐set DOC150 traps in double‐entrance tunnels had a higher

rate of captures. The models control for the fact there are effectively two DOC150 traps in the

same housing and there is a significant effect in the winter and autumn models. These trap

types also caught more stoats in the summer, and probably there is still an effect, but the lack

of captures overall means there is too high a standard error and the term falls out of the final

model. In addition, during the winter trial, there were three occasions when two stoats were

captured in the two traps within a double‐set DOC150 in double‐entrance tunnel. This is likely

due to a preference of the box type, where the elongated box with a tunnel structure appeals

to the inquisitive nature of stoats. This shows that this trap box type has a higher chance of

catching stoats.

In the summer models, despite a real lack of power, hen’s egg as a bait type showed a

considerably higher capture rate, and was probably the most useful term in the summer

models. In autumn, the hen’s egg bait was not useful for capture, and in fact the models

suggest this had the lowest capture rate. Only 2 stoats were caught on hen’s egg bait during

the autumn. In autumn, we saw significantly higher rates of capture with the smoked fish

bait in comparison to all other baits. Therefore, seasonality could be an important factor in

bait use.

Heathland has higher capture rate in winter and autumn, though this was not the case in the

summer models. It would be useful to have had a better habitat variable than ‘fields’, for

example, distinguishing between coastal grassland, rough in‐bye land and pasture. Also, it

would be worth marking if the traps are along some kind of boundary or linear feature, and

what this is.

Stoat gender was not able to be tested for due to lack of data. Many stoats were too

decomposed to be able to sex and therefore the dataset was much reduced.

Combining all seasonal data into one model would be worth trying, but since baits were used

inconsistently we would not be able to ask the models the full suite of questions.

Recommendations for TAG:

We would like the TAG to agree that:

‐ The ONWP should mostly use double‐set traps in double entrance tunnels for the

eradication.

‐ A mix of baits and lures should continue to be used and trialled through adaptive

management. Egg as bait should be used in spring and smoked fish in autumn.

‐ Trap placement should focus on heathland, rough unimproved grassland and coastal edge.

‐ All trapping data collected to date and into the future should be reviewed for stoat capture

preferences and the trapping operation adapted accordingly.

Appendix – Raw Stoat Catch Data: Summer and Autumn trials

NB: In the ‘stoat gender’ column, 0 refers to the fact that it was not able to be recorded mainly due

to decomposition.

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